In communication systems, particularly in receiver equipment wherein information is transmitted using FM signalling techniques, such as found in video transmission systems, various schemes have been developed to improve upon the input signal and carrier to noise ratio at which the equipment can deliver a useful output. These schemes have incorporated frequency modulation feedback and phase-locked loop receivers, and threshold extension applications involving the use of direct tracking filters that are tuned in response to the output of a phase comparator. Examples of demodulator configurations using these approaches are describe in U.S. Pat. Nos. to Wason 3,742,361; Tillotson 3,231,822; Webb 3,517,268; Colaguori 2,976,408 and my own previously developed improvement described in U.S. Pat. No. 4,087,756.
In those arrangements wherein tracking filters are employed, the precision and accuracy with which the tracking filter itself is able to follow the instantaneous frequency of the FM input signal over a relatively wide bandwidth can have a substantial impact on the overall performance of the receiver system.